The present invention relates to a single power source operated dual slope A/D converter with an automatic zero compensation circuit.
In a conventional hybrid circuit of a dual slope A/D converter and bipolar and MOS transistors, in order to guarantee stable operation of the A/D converter even in a low-voltage operation range, a threshold voltage of MOS transistor switches (to be referred to as MOS switches hereinafter) constituting switches for the A/D converter must be decreased. When the threshold voltage is decreased to, for example, about 0.2 to 0.5 V, a tailing phenomenon occurs wherein a current exponentially changing with respect to a gate voltage flows even at a voltage lower than a specific threshold voltage of a MOS transistor. Even if the gate voltage is set to be zero in order to turn off in the second step (an integration duration at an input voltage) a MOS switch of the automatic zero compensation circuit which has kept on in the first step (automatic zero compensation duration) of A/D conversion, a capacitor is discharged through this MOS switch. A noninverting input terminal voltage for an operational amplifier constituting the integrator is linearly decreased in the negative direction, resulting in A/D conversion errors.